The effect of infiltration and ventilation

by Adriaan Woonink

How can we reduce the energy needed to keep a building comfortable? What factors play a role? In this article we will explain how to reduce the energy consumption of buildings by reducing heat loss through infiltration.

 

Ventilation and infiltration

In essence, ventilation and infiltration are the same type of process in which internal air is replaced by external air. However, infiltration occurs at random through cracks and gaps in the building structure, whilst ventilation is the result of (mechanically) forced replacement of internal air. Infiltration can therefor be seen as uncontrolled and sometimes unwanted ventilation.

 

Having fresh air in a building is essential for the comfort and health of the building users. Buildings with low infiltration and ventilation can have a huge build-up of CO2 inside, causing discomfort and sometimes even sickness. A high infiltration rate of a building in essence is not bad for the health for the building users, but it increases the energy consumption to maintain the indoor temperature at comfortable levels. In the ideal situation a building has low infiltration and good ventilation rates. High ventilation rates may not be necessary and could still cause an unnecessary energy loss. This depends on how the building is used and how many people are in that building at a given time.

 

Quantifying heat loss

In order to quantify the heat loss of a building through ventilation and infiltration the concept of volume air change per hour needs to be defined. If we take the dimensions of a house as stated below we see the total building volume is 240m3. If the entire volume is replaced in 1 hour by ventilation we call this one volume air change per hour, or ACH. If the volume air change rate is 0,5 it means that half of the volume of air within the building (i.e. 120m3 in this example) is replaced by external air within one hour.

 

Length 5 m1
Width 8 m1
Height 6 m1
Floors 2
Floor area 80 m2
Volume 240 m3
Wall area 156 m2
Window area (30%) 47 m2
Floor area 40 m2
Roof area 40 m2
Envelope area 236 m2

 

We can now define ventilation heat loss rate as:

Qv = N * V * (Ti – To) * (c * ρ / 3.600)

Where:

Qv – ventilation heat loss rate (W)

N – volume air change per hour (h-1)

V – volume of the building/room (m3)

Ti – internal air temperature (K or ˚C)

To – external air temperature (K or ˚C)

c – specific heat of air (J/(kg*K))

ρ – density of air (kg/m3)

3.600 number of seconds in an hour

 

As (c * ρ / 3.600) is approximately 1/3, the above equation becomes:

Qv = (N * V / 3) * (Ti – To)

 

Assuming an inside temperature of 20˚C, an outside temperature of -10 ˚C and an N or ACH of 1 the equation becomes:

Qv = (1 * 240 / 3) * (20 – -10) = 2.400W

 

Air permeability

Different countries have different ways of expressing the maximum allowed air permeability, or the maximum allowed air infiltration in buildings. These values can differ per building type or size, but essentially all allow a maximum on the allowed air changes per hour in a building.

 

Air permeability can be expressed as a function of the total wall surface area of a building, or as a function of the total floor area of a building for example. It can also be simply expressed as a maximum allowed air change per hour, disregarding the shape or form of the design altogether.

 

To give an example, the CIBSE Guide A (CIBSE, 2015) gives the empirical relationship between air permeability at 50 Pascal in (m3/h)/m2 and infiltration rate in h-1 for a range of different building sizes and types.

 

The theoretic air permeability can for example be assumed to be 0,25. This needs to be multiplied with the volume and then divided by the external surface area of the building.

AP = 240 * 0,25 / 236 = 0,2542 / hr

 

If we establish through airtightness testing that the building envelope has an air permeability of 5 (m3/h)/m2 under a testing pressure of 50 Pa we can calculate the volume air change factor.

5 / 0,2542 = 19,67

 

Given these values we can calculate the air change rate of the building with the following formula:

N = AP * Aenv / V / 19,67

 

When we fill in all the variables we see that the initial assumption with an air permeability of 0,25 was quite accurate. Of course this will vary for each building separately.

N = 5 *236 / 240 / 19,67 = 0.2499/hr

Adriaan Woonink

Related Articles & Papers